The present invention relates to xerographic reproduction machines, and more particularly to improving the transfer of an image from a photoreceptor to a transfer member such as a sheet of paper.
In electrophotographic reproduction, a photoreceptor comprising a layer of photosensitive insulating material affixed to a conductive substrate is used to support electrostatic latent images. The surface of the photosensitive material is electrostatically charged, and exposed to a light pattern of an image to be reproduced to selectively discharge the surface in accordance with the image. The undischarged areas of the surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. The toner image is then transferred to a suitable substrate (e.g. paper), and the image is affixed thereto to form a permanent record of the original document. The process is well known, and is useful for light lens copying from an original, and printing applications from electronically generated or stored originals.
During the toner transfer, a substrate or transfer member such as a sheet of paper (hereinafter sheet or copy sheet) is caused to move in synchronized contact with the photosensitive surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the sheet remote from the photosensitive surface to electrostatically attract the toner image from the photoreceptor surface to the sheet. The copy sheet may be fed to the transfer station from a supply, a manual bypass path, or a duplex path to an area where it will contact the photoreceptor. To provide for an even transfer of toner material to the sheet, without undesirable dispersions or deletions caused by failure of toner to reach the copy sheets at the position corresponding to the original image, it is necessary that the copy sheet intimately contact the photoreceptor smoothly, without folds or wrinkles which, by causing surface variations in the copy sheet, cause transfer inconsistencies, and accordingly poor image quality.
To transfer the toner from the photoreceptor to the copy sheet, intimate contact between the two is required. However, during movement from the copy sheet supply through sheet feeding nip rolls and through a transfer station paper guide, or baffle arrangement to the contact position with the photoreceptor, the flimsy sheet tends to ripple, and there is a tendency for the side edges of the sheet, parallel to the direction of sheet travel, to contact the photoreceptor before the center portion of the sheet. As the photoreceptor is electrostatically charged, the sheet side edges tack to the photoreceptor somewhat tightly, and the sheet does not naturally smooth itself to form the required smooth intimate contact with the photoreceptor necessary for good image quality. In heavier, less flimsy paper weight sheets, the described problem is less noticeable as the heavier paper has a greater beam strength, which maintains the respective desired orientation of the edges and central portions of the sheets to allow appropriate contact. The problem of sheet non-smoothness is a particular problem with sheets returning from a duplex path having been subject to heating and extensive handling.
As the sheet leaves the transfer station baffle, there is a tendency of the trail edge of the sheet to flip away from the baffle due to a combination of factors including spring forces on the paper induced by the change in direction of the sheet as it moves away from the transfer baffle to contact with the photoreceptor, and the energy stored in the curled sheet releasing as it departs from the baffle. As a result of these forces on the paper, the trail edge of the sheet is caused to move suddenly towards the photoreceptor upon leaving the control of the transfer baffle. In this case, transfer of toner from the photoreceptor to the sheet is believed to occur before the trail edge of the sheet has made intimate contact with the photoreceptor, a phenomenon known as "gap transfer", causing a type of toner deletion known as "trail edge deletion".
U.S. Pat. No. 3,984,183 to Maksymiak suggests that increasing beam strength in a sheet improves post transfer stripping characteristics. Complementary curved photoreceptor and guide members accomplish this end by creating a curve extending in the sheet parallel to the direction of travel. The arrangement of curved guide members serves to provide a curve in the sheet matching a curve on the photoreceptor, whereby beam strength of sheets is increased. Increasing beam strength of sheets by applying a bow in a direction parallel to the path of paper travel is known in the printing arts as shown, for example, by U.S. Pat. Nos. 3,516,657 to Knudsen; 4,204,672 to Grivet; and 4,350,332 to Knight. These patents do not, however, teach applying a curvature to the sheet to induce a portion of the sheet to contact the photoreceptor prior to another portion.
U.S. Pat. No. 3,820,889 to Nitanda teaches a paper guide arrangement for avoiding certain deletion problems, including a pair of parallel spaced guide plates for directing transfer material to the photoreceptor. Either or both of the parallel guide plates are provided with a crowned edge in the plane of the guide plates.